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Unrecognized tax benefit (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]    
Beginning Balance $ 1,539 $ 1,314
Increase/(Decrease) of unrecognized tax benefits taken in prior years (126) 6
Increase/(Decrease) of unrecognized tax benefits related to current year 62 219
Ending Balance $ 1,475 $ 1,539